Resonance enhanced second harmonic generation (SHG) was used to examine the effect of surface charge on adsorption to silica/aqueous interfaces using para-nitrophenol (pNP).   pNP adsorption to silica surfaces from an acidic solution (pH = 1,5) is characterized by SHG spectra showing a single excitation wavelength (310 nm) that is measurably blue-shifted  from the bulk aqueous limit (at 318 nm).  ?Gads of pNP to the silica/aqueous (pH 1,5) interface is ~25 kJ/mole. At higher pH (pH > 7), SHG spectra show two excitation wavelengths at 310 nm and 330 nm that are interpreted in terms of pNP sampling two different solvation environments.  The longer wavelength species is assigned to pNP sharing its proton with a deprotonated �SiO- surface group. Using SHG field strength as a measure of surface population, both species have ?Gads values of ~22kJ/mol and these adsorption energies appear to be independent of pH (above pH 7).

Nonlinear studies of adsorption to polar and charged solid/liquid interface

245th National Meeting (2013)

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